Cyclosporine A enhances total cell calcium independent of Na-ATPase in vascular smooth muscle cells

The effect of cyclosporine A in enhancing vasconstrictor-induced calcium (Ca²⁺) mobilization in vascular smooth muscle cells may contribute to important side effects in cyclosporine therapy such as hypertension and nephrotoxicity. As we have previously shown, cyclosporine A stimulates transmembrane Ca²⁺ influx. Since Ca²⁺ efflux was not affected by cyclosporine A, we concluded that cyclosporine augments angiotensin II induced Ca²⁺ mobilization in vascular smooth muscle cells by an increased amount of Ca²⁺ in angiotensin II sensitive intracellular Ca²⁺ stores. The present study was therefore designed to examine the effect of cyclosporine A on cellular calcium content and on membrane calcium transport mechanisms. An important mechanism of Ca²⁺ extrusion from the cell is the Na-Ca exchanger. Its activity is closely related with that of the Na-ATPase. By increasing cellular sodium concentration the blockade of Na-ATPase would in turn activate cellular calcium uptake bx the Na-Ca exchanger. Therefore, we hypothesized that cyclosporine A might exert its effects in the same manner as a circulating Na-ATPase inhibitor. Total cell calcium was measured by atomic absorption and activity of Na-ATPase was estimated by an assay measuring phosphate production. Preincubation of the cells with cyclosporine (10 g/ml) for 15 min increased total cell calcium from 31.4 ± 5.0 to 46.5 ± 5.3 nmol/mg protein (P < 0.05). Activity of Na-ATPase was not affected by cyclosporine A (3.9 ± 0.2 vs. 4.3 ± 0.2 ol Pi h^–1 mg^–1 protein). Therefore, cyclosporine A induced Ca²⁺ influx is not mediated by an inhibition of the Na-ATPase. Cyclosporine-stimulated accumulation of cellular calcium may be mediated, for example, by opening of calcium channels in the plasma membrane. Increased Ca²⁺ mobilization in the presence of cyclosporine A may be due to an increased amount of Ca²⁺ avaible from intracellular Ca²⁺ stores. These results are of substantial significance for understanding the pathophysiological mechanisms of cyclosporine A induced vasoconstriction.Abbreviation CyA cyclosporine ACorrespondence to: H. Meyer-Lehnert